Blood flow rate measurements are taken in blood vessels, such as coronary arteries. In one blood flow rate measurement technique, Fractional Flow Reserve (FFR) may be calculated across a stenosis. FFR is defined as the ratio of the maximal blood flow achievable in a stenotic macro-vessel to the normal maximal flow in the same vessel. Such a measurement represents the fraction of the maximum flow that can be maintained despite the presence of a stenosis. In another blood flow rate measurement technique, absolute blood flow rate through a body vessel may be calculated (e.g., with a thermodilution system or method). The calculated absolute blood flow rate may be used for the diagnosis and understanding of microvascular disease.